Citizen Scientist Article

(Directly received satellite image of the Eastern U.S. and Atlantic Ocean. Click image to enlarge.)

Mark Twain once quipped, "Everyone talks about the weather, but nobody does anything about it!" Although there isn't much we can do about weather, we can increase our knowledge and awareness of it.

There are several ways to do this. We can stay glued to the television watching the Weather Channel. We can watch or listen to every local television or radio weather forecast. We can look at various weather sites on the Web. Or, at the very least, we can simply look out the window.

Amateur scientists have another option. We can supplement local weather news and web satellite images with real time satellite images that we receive when weather satellites pass overhead. This will provide a "Big Picture" view of the weather. By this I mean we can combine information from what we see and from local Doppler radar displays with real-time satellite data that is available more often than that posted on the web. This will provide a broad perspective of regional-scale events that will eventually influence local events.

In this article I will describe how to receive images directly from the many orbiting weather satellites designed specifically for this purpose. These satellites are launched and maintained by NOAA, the National Oceanographic and Atmospheric Administration. If you live in the Northeastern United States like I do, the NOAA 12, NOAA 15 and NOAA 17 polar orbiting satellites zip overhead several times a day making real-time observations of weather.

Many professional meteorologists have been using this real time data to create and perfect their computer models to determine if it will be rainy of sunny. What few people realize is that these data are available--in real time--to amateur scientists with little or no knowledge of electronics and orbital mechanics of satellites.

Many web sites and freeware and shareware programs indicate the position of overhead satellites based on the latitude and longitude of the observer. From the observer's coordinates, and a knowledge of the satellites orbital elements, the coordinates of the passing satellite, and the time that it will be overhead, will be provided. The weather satellite orbital element data are provided by several sources and can be automatically imported into different programs.

For those not familiar with orbital elements, here is a brief description. To determine the position of a terrestrial orbiting body, we need to know the shape of the orbit (ellipse), the size of the orbit, the inclination of the orbit, the orientation of the orbital plane, and the position of the orbiting body at a specified time. These points are commonly referred to as "Keplerian Elements" after the famous astronomer Johann Kepler. Depending on what convention you use, a minimum of six or seven points are needed to fully describe the elliptic orbit. It really is six points, but I will use seven, the convention used by WXtoImg, a satellite receiver program that I will introduce shortly.

The seven numbers will enable a mathematical ellipse to be placed in a simulated orbit around the Earth, along with its proper orientation in three-dimensional space. A knowledge of these numbers will enable an observer to determine when a satellite will be at a specific position in the sky at a specific time. The seven orbital elements are:

• Epoch
• Orbital Inclination
• Right Ascension of Ascending Node
• Argument of Perigee
• Eccentricity
• Mean Motion
• Mean Anomaly

These seven numbers will allow you to determine where and when an orbiting satellite will be at a specific time. To increase the accuracy of the prediction, several additional points can be added, such as atmospheric drag for low Earth orbiting Satellites. Since this is not a course in celestial mechanics, I'll leave the details of these orbital elements to be covered in detail by others more qualified to do so.

A series of PDF articles that give a wonderfully detailed explanation of orbital elements and celestial mechanics is available on the FTP Internet site orca.phys.uvic.ca/~tatum/celmechs . Another good source of information available from AMSAT, the Radio Amateur Satellite Corporation, is the "Keplerian Elements Tutorial." A good paperback reference is "Fundamentals of Astrodynamics" by Rodger Bate, Donald Mueller, and Jerry White (Dover).. NOAA , NORAD and NASA are also gold mines of information on the subject. Fortunately we don't need to know all this information to get a real-time weather image.

A higher resolution digital data stream is sent by the satellite's High Resolution Picture Transmission (HRPT) mode in the gigahertz range. For those who want higher resolution images, it is possible to put together some specialized equipment to receive the HRPT data. Details on how to accomplish this can be found at various sites on the Web.

Returning to the APT format, one can see that, without much effort, high quality images can be realized. Each NOAA (Eastern United States) satellite will have either a 137.50 MHz or 137.62 MHz FM signal. The APT format is a simplex (one-way) method of transmission with start and stop tones to facilitate unattended image data reception. NOAA has detailed specifications on their web site. During each satellite overpass, the signal will provide from seven to thirteen minutes of data. The total amount of data is dependent on the satellite's position relative to the observer's retriever. The NOAA Polar Orbiters are in a Sun synchronous orbit, meaning that they will pass overhead at approximately the same time of day.

Here are some operational NOAA satellites and their transmission frequencies:

NOAA 17 APT 137.62 MHz HRPT 1707.0 MHz

NOAA 15 APT 137.50 MHz HRPT 1702.5 MHz

NOAA 12 APT 137.50 MHz HRPT 1698.0 MHz

My first attempts to receive satellite signals in real time were met with disappointment. I connected the audio output of a Radio Shack hand held scanner (model PRO-64) to the LINE-IN connection on my computer's sound card. The antenna was the standard "rubber-duckie" that came with the receiver. I had no idea how strong the signal would be, so I tried this minimalist configuration first. The result was barely legible images buried in noise.

As any amateur radio ham or RF guy knows, one of the most critical components of any receiver is the antenna. To misuse a popular quote, an ounce of antenna is worth a pound of receiver. The fact that I live in a condominium complex also means that my antenna setup must remain indoors, eventually to be permanently mounted in the attic. Couple this with the limited sensitivity of the PRO-64, and its narrow IF bandwidth, and things don't get any better. I tried adding a Radio Shack magnetic mount mobile scanner antenna, but this made things only slightly better. The images were still buried in noise.

Hamtronics sells very specialized satellite receivers. I finally purchased an R-139 weather satellite fax receiver and a 137-MHz pre-amplifier for the antenna. The results were very impressive, but only when the satellite passed almost directly overhead. With the current configuration, the antenna seems to be the weakest link! I plan to experiment with either a high gain Yagi antenna or a QHA quadrafilar helix antenna . The only drawback of the Yagi antenna is that it is directional, so a modified television rotor assembly would be required to get the most bang for your buck.

If any of you are thinking of using an old television antenna, it will probably not function well. Television aerials are optimized for television frequencies, not the 137 MHz signals we are interested in receiving. The quadrifilar helix antenna has the advantage of giving horizon to horizon reception. If you are up to the challenge of making a tracking Yagi setup, the results might be worthwhile. Many web sites cover weather satellite antenna designs and tracking mechanism for Yagi antennas, so I will not re-invent the wheel by discussing them in detail here.

To get your system up and running, simply connect your antenna to your receiver (scanner, ham set or weather satellite receiver), and connect the receiver's audio output to the LINE-IN input of the computer's sound card. Once you are connected, run the WXtoImg program for the first time, and enter your geographic coordinates. If you have a GPS receiver, you can enter very accurate coordinates into the program. If not, the program includes a database with the coordinates of many large cites.

Once your ground station coordinates are entered, you should update the Keplers. As stated before, the "Keplers" are the set of orbital elements for each satellite. The Keplers are in the NASA 2-Line format , and should be updated on a daily basis. The reason for constantly updating is to make sure that the map overlays correctly match the images and that the calculated times for the satellite overpasses are accurate.

It is very important to keep your computer's real-time clock as accurate as possible. Several good freeware and shareware programs are available on the web to do this automatically. If your real time clock is off by more than a second, then the map overlay will be slightly off.

The Keplers need to be constantly updated, because the Earth is not a perfect sphere and is not perfectly smooth. There is an inherent "lumpiness" associated with the surface of the Earth which leads to slight gravitational irregularities. These irregularities will perturb the orbits of LEO (Low Earth Orbit) objects. The more up to date your orbital elements are, the more accurate your images will be. Here is a portion of a text file used to indicate the orbital elements or Keplers:

This file contains the orbital elements needed to determine the complete orbital path of satellites. For a detailed description of what each number means, visit the AMSAT web site. There you will find a good description of the file formats for various programs.

Getting back to the WXtoImg program, the next step is to determine when a satellite will be passing overhead. To do this, simply go to the File menu on the toolbar, and select Satellite Pass List. This will automatically determine the times when satellites will be in view. Bear in mind that you will have only up to twelve minutes of recording time for each overpass.

One of the nice features of the WXtoImg program is that it can be programmed to automatically record when satellites are in view. If you have a programmable scanner, you can program it to tune to the correct satellite frequency for different times of the day and night. The R-139 I am using has a six channel scan mode, which will automatically lock onto the frequency based upon the squelch setting. This enables a more hands free, automated setup.

I have set WXtoImg with options that begin recording when the satellite is overhead. Because of this, my S/N (signal to noise) ratio will be lower. I chose to do this to get the most coverage, although the upper and lower portion of the image will be noisy. You can play with the recording options to obtain better results. The recording options I have been using are shown in the screen image below: